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'load lines for dissimilar output tubes?'
Author:CChurchill (registered user: 163 posts )
Date: Wed, Mar 15th, 2017 @ 23:19 ( . )

On 03/15/2017 @ 16:45, tapehead ted wrote :
Hello all. I'm working on a design with a pair of 6L6 and a pair of 6v6 in parallel, in p-p amp, and trying to optimize the load lines. I will probably have a custom OT made, so there's the possibility of having a separate taps for each type of output tube, but this may not be necessary.
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: It seems to me that this is not as simple a case as paralleling two like tubes- Let's say I'm using a 4K OT. With four 6v6's, I would have a nice clear cut class B load line (Class AB overall) of 2K. But using dissimilar tubes, unless the current is the same in both the 6v6 and 6L6, I am thinking I will end up with a different Class B line for each tube type. This, I am thinking, will apply regardless of whether or not there is a separate OT tap for each tube type.
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: So how would one determine what the class B loadline would be with dissimilar currents, which I think is really all that is dissimilar about these "dissimilar" tubes when it comes to plotting loadlines?



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: (I'm using separate screen supplies to let me find more compatible operating points.)
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: I'm thinking this can be resolved using V=IR:
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: Say the 6L6 is biased to 60 ma, and the 6v6 to 40 ma. My thinking: At these currents, adding the 6v6 to the 6L6 is like adding 2/3 of another 6L6, not a whole other 6L6, and from the perspective of the 6v6, adding the 6L6 is like adding a 6v6 and a half, when it comes to determining the resulting loadlines.
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: Doubling the number of tubes, with the same currents, from the perspective of two 6v6s (40 ma each) adding two 6L6s (60 ma each) would be like adding three 6v6s. This I found a little easier to contemplate, and as far as I can tell the result is the same.
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: Is this how this works? Or is it at least close enough to how it works to make useful guesstimations?
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: My results show me that at these currents, a 4k OT looks like 10k to the 6v6s (for a 2.5k class B loadline) and the same 4k OT looks like 6.67 to the 6L6s (for a 1.67 class B loadline). This actually makes the tubes more compatible, offering more appropriate load lines to each, if in fact this thinking resembles reality.
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: Does it?
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: It seems like with dissimilar currents, the 6v6 and 6L6 couldn't be both on 2k class B loadlines with a 4k OT.
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: I'm focusing on the class B loadline as it is the one that crosses the 0 vg1 line, where screen dissipation becomes a serious issue, especially for the 6v6 with it's very small screen dissipation limit.
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: Any thoughts?
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: Thanks!
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So, this is an interesting question. I've though about doing this in passing (but not researched it). I wanted to touch on a couple of things you mentioned, but I'm sure you will get some more informed responses from those with expertise in magnetic coupling.
I'm not sure if I misunderstood you, but it sounds like you might misunderstand load impedance in the output stage somewhat. (please correct me if I'm wrong)
To oversimplify this, apart from the inherent tube internal resistance, an output transformer "reflects" the load impedance from the load - ie. speaker/resistor - to the tube. Often, when you look at the tube datasheets, you will find several different load impedance examples based on different operating conditions.
To your point, a 4K primary impedance with the same load would present a 4K load to any tube. Unless you were using different taps and maybe that's what you were talking about. An output tube doesn't "have" fixed output impedance under all conditions. However, there are popular choices which we often see used with different tubes.

However, I would think that the different operating characteristics of the dissimilar in a single primary winding (even with different taps) the tubes would cause instability and maybe damage to the transformer. I don't know for sure, but here is my guess and how I would approach it. I would use a separate push pull primary winding for each type of tube on the same core, as long as the transformer was designed to handle the stress and power of the two different tube types. This is an intuitive guess, there may be a reason it might not work because I've never seen it done this way, but this is how I would proceed.

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Replies:
--* load lines for dissimilar output tubes?
3/16/2017 @ 16:41--tapehead ted
3/17/2017 @ 09:23----Malcolm Irving
3/17/2017 @ 14:17------CChurchill
3/17/2017 @ 14:20--------CChurchill
3/17/2017 @ 14:54--------Malcolm Irving
3/17/2017 @ 19:51----------CChurchill
3/18/2017 @ 13:49------tapehead ted
3/18/2017 @ 15:21--------Malcolm Irving
3/18/2017 @ 15:35----------tapehead ted

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